Roof over - when new shingles are applied on top of existing shingles. The reason that customers ask question is that it can save time and money, however, it is NOT advised.
#1 Bumpy Surface
Shingles are meant to be installed on a flat, even surface. When installing on previous material, you're installing on an uneven surface that makes the shingles more difficult to install properly.
#2 Improper Inspections
Installing over existing shingles prohibits the roofer from adequately inspecting the condition of your roof and the plywood underneath. Without this inspection, your roofer can't ensure that you're beginning with the proper foundation and that could lead to costly repairs later.
#3: Lack of Leak Barriers
Overlaying shingles prevents roofers from properly installing ice and water barriers to prevent leaks into your home. In short, a roof over doesn't prevent or solve a problem, it just covers it up for a period of time.
#4: Weight Strain
Your roof is most likely built to withstand up to a specific amount of weight. A roof over just adds to the existing weight and will increase the strain on the trusses and could cause future problems.
#5: Voids The Warranty
Installing new shingles on existing ones will void the warranty on your new roof. This is because the old shingles can cause issues that affect the lifespan and performance of the new ones.